1. Field of the Invention
This invention belongs to the field of using images obtained by at least two strips of a scanning observation instrument mounted on board a vehicle (airplane, drone, helicopter, satellite, etc.) flying over a celestial body, such as the Earth.
2. Description of the Related Art
A strip of a scanning instrument comprises a plurality of sensitive cells (typically from 100 to 100,000) generally arranged in line and possibly on several lines. The line of sight of the strip is pointed toward the Earth's surface, and forms a plane with the line of sensitive cells, called “plane of sight”.
The instrument's main line of sight can be defined either as the line of sight of the central pixel of one of the two strips or as the line of sight of the central pixel of a fictitious strip that would be located, for example, at an equal distance from the two strips.
During image capture, each strip acquires successively images rows representing different linear portions of said scene, as its plane of sight scans the observed scene. Each sensitive cell of the strip records in a pixel the luminous intensity, within the wavelength considered, of an area of the linear portion aimed at.
The scanning of the plane of sight of each strip is achieved, for example, by moving the vehicle. This is generally the case for instruments on board a low-orbit satellite.
In general, the acquisition of images rows of a single strip is performed during a fixed time period (duration of acquisition), and with a substantially constant frequency of acquisition.
For a typical spatial application, an observation instrument is on board a satellite in orbit at 500 to 800 kilometers altitude; its line of sight is nominally pointed toward the center of the Earth; the instrument's focal length is between 0.5 and 15 meters, the field of view is between 1 and 20 degrees, each strip comprises a line of 5,000 to 50,000 sensitive cells, the scanning speed is between 6.5 and 6.8 kilometers per second, the acquisition frequency is between 500 and 5,000 Hertz; the swath (length of the linear portion covered by one image line on the Earth's surface) is between 10 and 100 kilometers, the dimension of an area imaged by one pixel is between 0.5 and 20 meters on the Earth's surface. The strips are generally 5 to 50 centimeters apart in the focal plane; the time gap between the image of a point on the earth's surface being captured by the first strip and its image being captured by the second strip is between 0.05 to 5 seconds.
The successive image lines acquired by a single strip form a pixel matrix called “composite image”, which represents the scene observed during image capture. It can be seen that, because the image lines are acquired at different times and because of the instrument's movements during image capture, the composite image distributed in a plane according to a regular grid does not represent the scene exactly as it would ideally be seen by an observer overlooking said scene. For example, because of these movements, a perfectly straight road may be curved in the composite image.
In the most general case, during image capture, two different strips will provide two different composite images whose pixels, acquired at a given time, represent different areas of the observed scene. In general, the layout of the strips, e.g. parallel in the focal plane of the observation instrument, is such that these composite images have a large amount of spatial overlap when the vehicle travels above the scene to be observed.
The differences between the two composite images are due to the fact that the lines of sight associated with the two strips are different. As a result, the composite images do not overlap completely at the beginning and the end of image capture. But the differences between the two composite images are mainly due to the movements of the instrument, since they can affect, at any given instant, different pixels in each composite image, specifically because of the gap between the two strips' lines of sight.
From patent FR 2 899 344, a method is known for reconstructing the line of sight, in which homologous characteristic areas between the composite images of a single scene acquired by two different strips are determined by correlation, and the angular variations of the line of sight are reconstructed according to the geometric distances between the positions (i.e. the pixel indices) of the characteristic areas in each of said composite images.
However, this method has many limitations.
For example, the ability to determine, or not, homologous characteristic areas in the two composite images very much depends on the nature of the observed scene. For example, for weakly textured scenes, such as sea/ocean and/or snowy mountain scenes, it will be impossible in practice to determine a sufficient number of homologous characteristic areas to achieve good performance in reconstructing the line of sight.
In addition, the method of patent FR 2 899 344 is sensitive to the effects of parallax and/or to the presence of clouds, which can distort or prevent the correlation of the homologous characteristic areas.
In the method of patent FR 2 899 344, the relative angular variations from one sampling instant to another are the parameters estimated by correlation between the homologous characteristic areas; this requires an additional step of filtering and integration to reconstruct the absolute angular variations, which step may be detrimental in terms of computation complexity.
In addition, the method of patent FR 2 899 344, which is very suitable for a pair of composite images, cannot be easily generalized to processing simultaneously more than two composite images with different time shifts or even different acquisition frequencies.